1. Field of the Invention
This invention relates to isolating cathodic protection of underground utilities while permitting signals, such as underground locating signals, to be transmitted along the underground utilities. Such signals include, but are not restricted to, radio signals.
2. Background Information
Cathodic protection of underground pipe including steel gas mains is a common practise to prevent corrosion in the utilities industries. This can be a galvanic protection system or an impressed current protection system.
In a galvanic protection system, it is a common practise to have a corrosion protection anode electrically connected to a metal gas main so as to provide galvanic protection to the metal gas main. In galvanic protection, a metal substance that is strongly anodic to steel, usually magnesium or zinc, is buried in the ground adjacent to the steel gas main and connected to it with copper wire. This establishes an electrochemical cell or battery in which the buried metal is the anode, the steel gas main is the cathode, and the soil is the electrolyte.
In an impressed-current protection system, a large mass of expendable metal or graphite, called a ground bed, is buried in the ground some distance from the pipe. This ground bed is connected to the positive side of a source of direct current and the pipe is connected to the negative side. The current source can be viewed as a pump for electrons with its suction side connected to the ground bed and its discharge connected to the pipe. As electrons are removed from the material in the ground bed, an electron-generating or anodic reaction occurs at its surface. The exact nature of the reaction depends primarily on the material of the ground bed.
In both types of cathodic protection, flow of current occurs through the soil, from the buried material to the pipe.
The anode deteriorates, but the metal gas main is protected from corrosion.
In original installations there are metal service lines from the metal gas main to customers' gas meters. As these deteriorate, it is a common practise to insert plastic gas service tubing through the deteriorated metal gas service lines. In the United States, it is a Dept. of Transportation requirement, (RE 49 CFR Ch.1 Part 192.321 e) to have a method of locating the plastic gas service tubing. A common way to do this is to have a tracer wire installed with the plastic tubing so signals such as radio signals can be transmitted through the tracer wire. When one is searching for the location of the plastic gas service tubing, one can send a signal, such as a radio signal, through the tracer wire while a person with a detector can detect the location of the plastic tubing.
A problem with using a tracer wire with the plastic tubing within a metal pipe is that if a short occurs between the tracer wire and the metal pipe, there is a direct electrical connection between that metal pipe and the gas main to which the tracer wire was connected. This causes the corrosion protection anode to deteriorate much faster, as it is now trying to protect not only the gas main, but also the metal pipe through which the plastic tubing was inserted. This is not desirable.
FIG. 3 illustrates a prior art solution to the problem of how to attach a tracer wire 2 to a metal gas mains 3 while keeping the gas main 3 isolated so shorts in the tracer wire 2 do not compromise the cathodic protection of the gas mains. Valve boxes 24 with covers 25 are used as check points, where sections of tracer wire 2 are isolated from other sections of tracer wire 2. A problem with this is that if one of the valve boxes 24 with a cover 25 is removed, plowed under, paved over, or otherwise lost, tracing the elements of a utility distribution system becomes more difficult. Also shown in FIG. 3 are the prior art steel to plastic transition mains 21 which are connected to the gas mains 3 by means of the welds 22. The steel to plastic transition mains 21 are then coupled to the plastic main 23, typically by a plastic weldment 26. The steel to plastic transition mains 21 are typically a steel pipe coupled to a plastic pipe and serve as a bridge between the metal gas mains 3 and the plastic main 23. Tackwelds 9 connect sections of tracer wire 2 to the steel side of the steel to plastic transition mains 21.
As will be shown in the subsequent description of the preferred embodiments of the present invention, these and other shortcomings of the prior art are overcome.